The fatigue characteristics of a welded structure are extremely important characteristics in determining the lifetime of a structure itself. As means for improving such fatigue strength of a welded structure, there are the methods of smoothing the shape of the weld toe to ease the stress concentration as much as possible or applying peening etc. to locally impart compressive residual stress at the places where fatigue occurs. Furthermore, as shown in the art described in PLT 1, the method of lowering the transformation start temperature of the weld metal and improving the fatigue strength by the effect of reduction of the residual stress by utilizing expansion due to transformation etc. have been disclosed.
However, in the prior art such as the art described in PLT 1, for example, the means for application to a joint where the weld toe is structurally sealed off was not disclosed.
FIG. 1 is a view showing one example of welded joints and a welded structure. This FIG. 1 is a schematic view for explaining the structure in the case of attaching a member having a U-shaped cross-section to a flat plate by welding so as to secure bending rigidity. In the case of the example shown in FIG. 1, the U-shaped member is welded with the flat plate at two locations. The joints are T-joints. At this time, fatigue cracks occur at the stress concentration parts, so in the example shown in FIG. 1, occur at the four locations shown by the symbols A to D. Among these, the two locations shown by the symbols A and B are positioned at the outside of the welded structure, so repair is easy. Further, it is possible to work the weld toes smooth in advance or perform peening to impart compressive residual stress so as to improve the fatigue strength.
However, the weld toes at the two locations shown by the symbols C and D in FIG. 1 are sealed off in structure, so cannot be post-treated after the end of welding. This is because of the extremely simple reason that in the case of peening or other such mechanical post-treatment methods, it is necessary to directly contact and treat the parts where fatigue would become a problem (see locations of symbols C and D in FIG. 1). Therefore, the fatigue strength of the welded structure shown in FIG. 1 is determined by the fatigue strengths of the weld toes shown by the symbols C and D. No matter how much the fatigue strengths of the weld toes shown by the symbols A and B are improved, the problem remains that the fatigue strength of the welded structure as a whole is not improved.
On the other hand, even in the art described in PLT 1 or 2, the art disclosed in the literature only discloses art for joints in the case where the weld toes are positioned at the outside. For example, in an actual welded structure, how to use the weld material disclosed in PLT 1 in the case where fatigue cracks occur in the weld toes positioned at the inside is not necessarily clear. In the case of FIG. 1, the T-joint is completed by two-pass welding, but in this case, the heat when forming the subsequent weld beads, that is, when forming the outside weld beads, cancels the residual stress formed by the inside beads, so the inherent effect cannot be obtained. On the other hand, if forming T-joints such as shown in FIG. 1 by single-pass welding, butt solidified weld results and hot cracks are liable to form at the weld zones. Further, when adding alloying elements to the weld metal to an extent which reduces the residual stress, the hot crack sensitivity becomes far higher than with an ordinary weld material. Art which avoids this problem while improving the fatigue strength has been considered necessary.